1. Field of the Invention
The present invention relates to a sheet binding device for binding sheets, a sheet processing apparatus, image forming apparatus, and image forming system including the sheet binding device, and a sheet binding method.
2. Description of the Related Art
Conventionally, there is known an image forming system including a sheet processing apparatus equipped with a sheet binding device that binds a bundle of sheets on which images have been formed by an image forming apparatus with a binding tool provided as a binding unit.
Japanese Patent Application Laid-open No. 2010-184769 has disclosed a binding tool for binding a bundle of sheets without using any metal staples in such a manner that the sheets are crimped by strongly engaging crimping teeth, which are a pair of crimping members having a toothing aligned in a predetermined direction, thereby entangling fibers of the sheets. The bundle of sheets is bound by crimping without using any metal staples; therefore, it is possible to avoid the trouble of having to remove metal staples from a sheet bundle when the sheet bundle is discarded or shredded.
The inventors of the present application have developed a sheet binding device that performs crimp binding of a sheet bundle by moving one of a pair of crimping teeth by means of a link mechanism and a crank mechanism, thereby pressing the sheet bundle with the pair of crimping teeth.
The sheet binding device has a pressing-force giving unit for giving a pressing force to press a sheet bundle inserted between the pair of crimping teeth, against the crimping tooth. This pressing-force giving unit includes a link mechanism that is connected to the lower crimping tooth and moves the lower crimping tooth in a vertical direction, a crank mechanism that operates the link mechanism, and a drive source that drives crank mechanism.
When the sheet binding device binds a sheet bundle, the crank mechanism is rotated by the drive source, thereby the link mechanism is extended, and the lower crimping tooth moves toward the upper crimping tooth. Accordingly, the sheet bundle is held between the upper and lower crimping teeth and pressed with the upper and lower crimping teeth; thus, crimp binding is performed on the sheet bundle.
In this sheet binding device, a rotating shaft of the crank mechanism is fixed to a device body; therefore, once a sheet bundle is held between the upper and lower crimping teeth, the link mechanism being in a flexed state is locked at the position. When the link mechanism is locked in this way, the crank mechanism cannot be rotated in the same direction anymore, so the crank mechanism is also locked.
Therefore, after the sheet bundle has been bound, the crank mechanism is rotated in a direction opposite to the rotation direction at the time of binding the sheet bundle to move the lower crimping tooth away from the upper crimping tooth so that the link mechanism and the crank mechanism are unlocked. Then, the lower crimping tooth is moved until the space between the upper and lower crimping teeth reaches a predetermined interval, and after that, the sheet bundle is taken out from between the crimping teeth.
However, the relative distance between the upper and lower crimping teeth when a sheet bundle is held between the upper and lower crimping teeth varies according to the thickness of the sheet bundle. Therefore, when the lower crimping tooth is moved until the space between the upper and lower crimping teeth reaches the predetermined interval after the sheet bundle has been bound, a rotation amount by which the crank mechanism is to be reversely rotated varies according to the thickness of the sheet bundle. Accordingly, it is necessary to acquire the thickness of a sheet bundle and perform control of reversely rotating the crank mechanism on the basis of a rotation amount preset according to the thickness of a sheet bundle and the acquired thickness of the sheet bundle, and there arises a problem that the control in binding becomes complicated.
Furthermore, the above-described problem occurs not only in the configuration in which the link mechanism is operated by the crank mechanism but also in a configuration in which the link mechanism is operated by a cam mechanism adopted instead of the crank mechanism.
In view of the above, there is a need to provide a sheet binding device, a sheet processing apparatus, image forming apparatus, and image forming system including the sheet binding device, and a sheet binding method capable of simplifying the control in binding regardless of the thickness of a sheet bundle.